The discharge vessel of high-pressure discharge lamps, in particular those filled with a metal halide, have a reflective coating at the ends. This coating regulates the heat balance and thus determines the constancy of the color temperature and the average life of the lamp.
Previously, suspensions containing silica sol and/or glass powder have been used as a paste for preparing reflective coatings. To adjust the paste to a suitable viscosity and prevent sedimentation, organic additives such as cellulose ether are added to the binder in relatively large quantities. However, during the manufacture of the lamp, these organic additives must be eliminated completely. If the additives are not completely eliminated, they leave a residue which makes the reflective coating look grey, and so do not enable optimal heat reflection of the kind a white coating provides. To eliminate the organic additive residue, however, requires baking of the coating for a long period, as well as an additional "white-burn operation" lasting several minutes, in which air or oxygen is added and the residues are oxidized.